No, do not combine goldfish and bettas. There are a few reasons for this; One, bettas are tropical fish and need warm temperatures while goldfish are coldwater fish. If the goldfish is a fancy goldfish, then it has a betta-like tail which may make the betta want to attack it. Like jooranda said, if you want tankmates for your goldfish you'll have to get more goldfish (depending on the size of your tank, of course).